In the present state of art, two totally different types of shoulder prosthesis are routinely implanted, namely the “anatomical” shoulder prosthesis characterized by the presence of a convex-shaped humeral head implant (cooperating with the glenoid cavity of the scapula) and the “reverse” (or “inverted”) shoulder prosthesis, characterized by the presence of a concave cup-shaped humeral implant (cooperating with a “glenosphere”, which is a hemispherical implant attached onto the glenoid cavity).
Treatment of complex upper humerus fractures in elderly people with osteoporosis is a frequent indication for prosthetic shoulder joint replacement. However, functional results after implantation of “anatomical” shoulder prosthesis on fracture are quite mediocre and inconstant because of a high rate of malunions (bone healing in a non-anatomical position) and nonunions (absence of bone healing) of the tuberosity bone fragments (greater tubercle and lesser tubercle) around the upper part of the prosthetic stem. These tuberosity bone fragments are not only osteoporotic and fragile but also sustain important strains because of movements in the adjacent shoulder joint and because of traction forces from the muscle insertions (rotator cuff) on these tuberosity bone fragments. Therefore, stable fixation of the tuberosity bone fragments onto the prosthetic stem is of utmost importance for optimal bone healing and functional recovery.
Tuberosity bone fragment fixation can be done by means of bone sutures going through smooth holes in the upper part of the prosthetic stem and/or by means of locking bone screws into threaded boreholes. Tuberosity bone fixation by means of bone sutures not only is a meticulous and tedious technique but is also relatively fragile with a high risk for the sutures to cut through the edges of the osteoporotic bone fragments, thus resulting in suture loosening and secondary displacement of the bone fragments. Bone fragment fixation with locking bone screws is more straightforward and is also mechanically stronger, especially if associated with tension band sutures going around the protruding heads of the bone screws.
In case of a bad functional outcome of an anatomical prosthesis on upper humerus fracture, secondary conversion of the anatomical prosthesis into a reverse shoulder prosthesis can be an option. In that case, the prosthetic humeral head implant has to be replaced by the concave cup-shaped humeral implant. Secondary conversion of the prosthesis with conservation of the already implanted intraosseous prosthetic stem is an important advantage, not only for the surgeon, but especially for the patient, because extraction of the already implanted (cemented or osteointegrated) prosthetic stem out of an osteoporotic bone is not only difficult and takes time for the surgeon but also can cause new complicated bone fractures with supplementary morbidity and impairment for the already fragile patient.
Furthermore, because of the mediocre functional results of anatomical shoulder prosthesis on fracture, many surgeons nowadays readily opt for primary implantation of a reverse shoulder prosthesis on fracture. Even in that case, stable tuberosity fixation onto the reverse shoulder prosthesis is still useful in order to obtain optimal recovery of active external rotation.
Therefore it is mandatory that a shoulder prosthesis, specifically designed for fracture treatment, has a “universal” stem, in order to allow implantation of a prosthetic humeral head (anatomical shoulder prosthesis) as well as a humeral cup (reverse shoulder prosthesis). In the present state of art, most existing universal shoulder prosthesis systems are equipped with an inverted Morse taper connection at the upper end of the stem, which means that the conical (male) Morse taper is protruding from the underside of the humeral head implant or from the underside of the humeral cup-shaped implant. Thus, the downwards orientated (male) Morse taper is impacted into the corresponding (female) tapering cavity situated on the upper surface of the prosthetic stem.
The inverted Morse taper connection is to be clearly distinguished from the standard Morse taper connection used in standard anatomical shoulder prosthesis systems. In the standard Morse taper connection, the (male) Morse taper is protruding upwards from the upper surface of the prosthetic stem and is impacted into a (female) tapering cavity on the underside of the humeral head implant. The presence of a standard (upwards orientated) Morse taper connection is totally incompatible with the concave humeral cup-shaped implant of the reverse shoulder prosthesis because of spatial obstruction.
Anchoring bone screw fixation into a threaded borehole across the prosthetic stem is not possible in the presence of an inverted (downwards orientated) Morse taper connection because of the spatial obstruction caused by the male Morse taper inside the upper part of the stem. Indeed, a Morse taper connection can be reinforced with an axial screw, which will reinforce the impaction of the tapering connection, but the Morse taper connection is totally incompatible with a transverse locking screw which would cause loosening of the impacted tapering connection.
In short: in the present state of art, locking screw fixation into threaded boreholes on the prosthetic stem, as described in patents U.S. Pat. No. 6,398,812 and WO2005089678 is only possible on an anatomical shoulder prosthesis with a standard (upwards orientated) Morse taper connection, but this type of prosthesis doesn't allow the surgeon to do primary or secondary conversion into a reverse prosthesis. Therefore, in the present state of art, many surgeons prefer to use universal shoulder prosthesis systems with an inverted (downwards orientated) Morse taper connection, thus losing the ability to use locking bone screws for tuberosity fixation.
In the technique of osteosynthesis with a medullary nail, it appears clearly that the bone screw holding the greater tubercle is best located at the level situated just above to the center of the (imaginary) circle surrounding the humeral head (see FIG. 2). The position of a horizontally orientated bone screw, situated above the center of the humeral head is referred to as the “supra-equatorial” position. In addition, the supra-equatorial bone screw should have a posterior-to-anterior radial orientation of about 30 to 45 degrees to the frontal plane, thus imposing the presence of two different holes for the same screw, depending on the side of implantation (right shoulder or left shoulder). In most of the actual types of prosthetic stems, especially in the “low bulk” stems for fracture treatment, the upper end of the prosthetic stem is very narrow with insufficient space for a (double) supra-equatorial locking borehole.
Fixation of a targeting arm onto the upper part of a prosthetic stem also creates a problem of spatial obstruction. Indeed, fixation of the targeting arm onto the lateral side of the prosthetic stem as described in patents WO2005089678 and EP2002794 precludes reduction of the tuberosity bone fragments around the prosthetic stem. Fixation of the targeting arm onto the upper surface of the prosthetic stem (passing through a hole in the prosthetic head) allows unhindered tuberosity fracture reduction but, in that case, the axial fixation screw of the targeting arm will create obstruction for a horizontally orientated supra-equatorial bone screw.
In the technique of osteosynthesis of fractures of the upper humerus with a medullary nail, the nail is inserted through the lateral part of the native humeral head, the upper end of the nail being located only a few millimeters underneath the cartilage surface. Therefore the junction between the nail and the targeting arm is located at the level of the humeral head, thus leaving sufficient space for a supra-equatorial bone screw anchoring within the upper part of the medullary nail, just beyond the axial fixation screw of the targeting arm. The technical solution which would consist in an elevated knob protruding from the upper surface of the prosthetic stem, in order to allow fixation of the targeting arm together with supra-equatorial screw fixation, would have the inconvenience that this knob would preclude secondary conversion of the anatomical prosthesis into a reverse prosthesis.
The technical solution proposed in patent WO2004024029, consisting of the fixation of a prosthetic humeral head directly onto a medullary nail, has the inconvenience that the presence of the claimed “lateral connection means” considerably reduces the available space for transverse locking bone screws. Furthermore, the lack of osteointegration of the medullary nail with persistent (micro)-movements between the nail and the surrounding bone will result in pain and functional impairment for the patient. This technical solution neither foresees the possibility of secondary conversion into a reverse shoulder prosthesis.
The technical solution proposed in patent FR2909858, consisting in driving bone screws directly into a polyethylene “connecting wedge” disposed around the upper part of a prosthetic stem has the inconvenience of reducing the metallic surface of the prosthetic stem and therefore will decrease the chances for osteointegration together with the additional risk that the micro particles of polyethylene, which are released during the drilling of the holes, might even induce osteolysis.
Patent WO9739693 claims a tri-partite prosthetic stem comprising a lateral part meant for bone screw fixation, in combination with an inverted Morse taper connection for the prosthetic head, seemingly neglecting the abovementioned problems of spatial obstruction.
Therefore, the technical problem to be solved is to allow locking of a bone screw in a supra-equatorial position onto a universal prosthetic stem without reducing the outer osteointegration surface of the stem.